The present invention relates to recording surfaces of magnetic disc drives. More specifically, the present invention relates to overcoats on recording surfaces of magnetic discs for use in magnetic disc drives and processes for making the same.
Computer systems, or the like, often times employ magnetic disc drives to store information such as computer programs or data. Magnetic disc drives typically include a transducing head mounted on a slider which "flies" over the surface of a rotating rigid magnetic disc. The transducing head is positioned over a selected portion of the disc by an electronic circuitry controlled actuator. The transducing head is used to generate magnetic fields which are impressed onto the surface of the disc during writing of information, and to sense magnetic fields from the disc surface during readback of information.
The disc includes a magnetic layer deposited on a rigid substrate. The magnetic layer is suitable for storing magnetically encoded information. When the disc drive is powered down, the transducing head rests on the surface of the disc. During operation of the disc drive, the transducing head typically is spaced apart from the surface of the disc by a layer of air under the slider which is created by the rotating disc. The surface of the disc "pulls" air under the slider which lifts the slider off the surface of the disc. Although the slider is lifted from the surface of the disc, it is known that improved magnetic interaction between the transducing head and the disc can be achieved by reducing the spacing between the transducing head and the disc surface. Therefore, the transducing head generally flies proximate the disc surface such that limited, intermittent contact occurs between the slider and disc surface, which causes wear of the disc surface. Wear causes a degradation of the disc which adversely affects its ability to store information thereon. Additionally, the disc is susceptible to corrosion which also provides detrimental effects.
In order to protect the disc from wear and corrosion, overcoats are usually deposited thereon. A suitable overcoat must not only resist wear and corrosion, but also maintain proper hydrodynamic effects of the disc surface, as measured by "stiction" and "friction", such that the slider and transducing head may efficiently interact with the rotating disc.
Overcoats are deposited as a film on the magnetic surface, which permits proper interaction between the magnetic surface and the transducing head. Such film overcoats are typically applied to the magnetic layer by a process known in the art as "sputtering". Simply put, sputtering requires the rigid disc having the magnetic layer to be placed in a process chamber proximate a negatively charged "target", such as carbon, which will eventually comprise the overcoat. An argon gas is provided to the process chamber. A high voltage is applied to the gas such that it becomes a plasma including positive charged ions, electrons and neutral argon atoms. The positive charged ions of the plasma bombard the target. The resulting substance is deposited on the disc, which is electrically grounded, as a thin film. In this example, such a thin film is known as amorphous carbon (a-C).
Amorphous carbon films were once popular overcoats in the magnetic recording medium industry. Amorphous carbon films provide a very hard surface which resists mechanical wear. However, amorphous carbon films severely suffer from a condition known as "tribochemical", wear, which is undesirable. The disadvantages of amorphous carbon films have caused it to fall out of favor in the industry.
Amorphous carbon films have largely been replaced with doped amorphous carbon films, for example hydrogenated amorphous carbon (a-C:H) or nitrogenated amorphous carbon (a-C:N), as overcoats. Hydrogenated amorphous carbon, for example, is hydrogen doped amorphous carbon. It can be produced by the sputtering process described above wherein the argon gas is mixed with a certain amount of hydrogen. As such, doped amorphous carbon film is produced by a process also called "reactive sputtering". Similarly, nitrogenated amorphous carbon is nitrogen doped amorphous carbon. It can be produced by reactive sputtering wherein the argon gas is mixed with a certain amount of nitrogen. Doped amorphous carbon is characterized by high electrical resistivity.
Although the doped amorphous carbon film is not as hard as amorphous carbon film, doped amorphous carbon film has a significantly higher resistance to tribochemical wear than amorphous carbon film. As such, it has been proven that the overall performance of doped amorphous carbon film is far superior to amorphous carbon film, and thus doped amorphous carbon film is currently very popular with the magnetic storage medium industry.
Doped amorphous carbon films, however, are not without disadvantages. A common problem with sputtering of doped amorphous carbon film is arcing (or sparks) from the carbon target which can cause damage to the disc. Such damage is commonly called carbon induced damage (CID). CID is undesirable because it can adversely affect the information storing properties of the disc.
Arcing is believed to originate from the development of positive charge at some local irregularities or foreign particles on the carbon target. When hydrogen/argon gas, for example, is introduced at the target, a polymer film comprising carbon and hydrogen can form on the target. The polymer film is an insulator, and therefore forms "insulating sites". The electric charge at the insulating sites cannot dissipate quickly enough and a positive charge accumulates at the insulating sites.
Because the target is under a negative potential during sputtering, a voltage is developed between the target and the positively charged insulating sites. When the voltage becomes sufficiently high, a dielectric breakdown can occur which causes large electric current density in the local area. In other words, local arcing occurs. Additionally, the high current density causes a very intense temperature rise, which can transform the carbon target proximate the insulating sites into a glassy carbon. The glassy carbon sites are undesirable and provide a suitable region for the deposition of backsputtered species. The glassy carbon sites become "nodules". Often times the top layer of the nodule is electrically resistive and a positive charge is developed. A voltage is developed between the positive charge and negatively charged carbon target which serves to prevent the surrounding area from being sputtered. When a sufficiently high voltage is developed, dielectric breakdown occurs. The dielectric breakdown serves to spill nodule pieces in the chamber. Some of the spilled nodule pieces hit the disc and cause CID.
It has been determined that the frequency of the arcing is related to the resistivity of the overcoat film. Hydrogenated amorphous carbon, for example, is much more resistive than amorphous carbon. Thus, arcing occurs much more frequently, and the problem of CID is much more severe with hydrogenated amorphous carbon, and also with nitrogenated amorphous carbon, than with amorphous carbon.
In order to address the problem of arcing associated with the use of doped amorphous carbon, the prior art employs a system which periodically cleans the targets of nodules and insulating sites. During cleaning, argon gas, rather than the mixture of argon and hydrogen gas, for example, is used to bombard the target to break up the nodules and insulating sites. The resulting amorphous carbon is quite conductive, which helps dissipate the charge accumulated on the nodules. During cleaning, the resulting amorphous carbon and residuals are exhausted from the chamber.
FIG. 1 shows the frequency of CID as a function of sputtering time using a hydrogen/argon gas mixture. CID frequency is dramatically decreased after argon cleaning, shown at P100. Thereafter, CID frequency increases with time to almost 40 percent after 20 hours of sputtering time, P102. Thus, in order to avoid CID's, the target must be cleaned quite frequently.
Unfortunately, cleaning the target occurs during down time, which, as described above, must occur quite frequently to avoid CID's. Also, the cleaning process typically requires about an hour of down time to effectively remove the nodules and insulating sites. Thus, each day, manufacturing time must be interrupted in order to effectively clean the target. Such interruptions significantly reduce throughput and manufacturing efficiency. Thus, there exists a need for an overcoat film which can effectively protect the magnetic layer of a disc and which can be deposited thereon by a process that can easily be implemented and does not produce significant manufacturing inefficiencies.